<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>

	<div id="app1">
		{{a}}
	</div>
	
	<script src="node_modules/vue/dist/vue.min.js"></script>
	<script>
		var app1 = new Vue({
			el: '#app1',
			data: {
				a: 1
			}
		});

		// app1.$data
		// app1.$el

		app1.$watch('a', function(n, o) {
			console.log(n + ':' + o);
		});
	</script>
</body>
</html>